Measurements of a web and a paper surface at the manufacturing stage are important to ensure that the surface, printability and quality of the end product, i.e. paper, in general meet the production demands.
The surface topography of ready-made paper or paper under manufacture may be measured in various ways. For instance, a Bendtsen measuring device according to the standard SCAN-P21:67 may be used to determine the surface roughness, whereby a sample is put onto a glass plate and a measuring head producing compressed air is set on top of the sample. The amount of flowing air is measured, and the measurement is repeated at least 20 times at different points of the paper sample. The rougher the sample is, the more air is flowing, and thus the surface roughness can be measured on the basis of the flow rate.
The surface topography has also been measured optically. In this case, the measurement can be carried out based on, for instance, light scattering or by employing photometric stereo photography.
Optical radiation, such as light, can be focused on the sample surface and the intensity of the scattered light can be measured from different directions. The intensity of the optical radiation scattered in different directions depends on the roughness of the surface.
In a stereo measurement, two images of the surface are formed, taken from different directions. By combining the images as one three-dimensional image, it is also possible to illustrate the surface variation in the depth direction. The size of surface irregularities and the direction of the shapes may thus be measured from the image.
These measurements are nevertheless related with problems. The Bendtsen measurement is slow and not suitable for an on-line measurement. Also, surface topography is not the only factor affecting the Bendtsen measurement, wherefore the measurement does not describe the nature or scale of the surface topography. Measurements of optical radiation scattering may well be proportioned to the Bendtsen measurement but neither the Bendtsen measurement nor the measurement of scattering describes the nature or scale of the surface topography.
A photometric stereo measurement, for its part, is slow and complex because, among other things, two images taken from different directions must be combined.